I assume the vehicle telemetry downlink and the comm link to the ISS are encrypted to avoid interference.

I know the Shuttle carried DOD Secure comm equipment in addition to it's in the clear UHF and VHF Radios. I'm assuming that ISS has something similar.

> I know the Shuttle carried DOD Secure comm equipment in addition to it's in the clear UHF and VHF Radios. I'm assuming that ISS has something similar.

There's some obvious reasons why the International Space Station might be less likely to carry sensitive US military communications gear than the NASA Space Shuttles.

I don't think it is anything like that. It's for cases where they need to discuss something that they don't want every person with a HAM radio to hear. A discussion between an astronaut and the flight surgeon/spouse/clergy member. I'm sure the Russians have a system on ISS as well. Just having the hardware is not enough to crack into an enemies comms. It's more that they don't want stuff like that falling into the hands of a rouge state like Iran or North Korea, or a non-state actor that might have schemes of reverse engineering it to build their own system. I'd bet there is a Russian Transceiver racked up in JSC, and a DOD system racked up at Russian Space Control.

I don't get why you would need secret hardware for that -- all the encryption can happen in software, and there are plenty of public known encryption algorithms that are secure. Maybe the need for secure storage of a private key?